1. Field of the Invention
The present invention relates to an exhaust emission control device for reducing a nitrogen oxide included in exhaust air of an internal combustion engine such as a diesel engine, and the invention is effectively applied to vehicles.
2. Description of Related Art
According to an exhaust emission control device for reducing a nitrogen oxide (NOx) included in exhaust air of an internal combustion engine such as a diesel engine, the nitrogen oxide is purified (reduced) by providing in an exhaust pipe a catalyst that promotes a reduction reaction and by injecting an additive agent such as a urea water solution into exhaust air flowing into the catalyst (see, for example, JP2003-293739A).
More specifically, urea (CO(NH2)2) injected into exhaust air is hydrolyzed by exhaust heat (CO(NH2)2+H2O→2NH3+CO2) to generate ammonia (NH3), which is a reducing agent. Then, the nitrogen oxide is reduced by reaction between the nitrogen oxide and the ammonia through the catalyst.
In the above-described exhaust emission control device, the nitrogen oxide in exhaust air reacts with the ammonia through the catalyst to be turned into nitrogen (N2) and water (H2O). Meanwhile, when the catalyst deteriorates, reduction ability is reduced, and the nitrogen oxide is not fully reduced. Accordingly, excessive ammonia (reducing agent) is caused, and thereby the consumption of the excessive additive agent is generated.